Fig. 4. Mechanism investigations of AC-MoRuO_x.

(A) Illustration of catalytic activity increases with the increase in the disordered phase. The black arrows represent the enhanced role of adatom or configuration change. (B) Top view of the possible doped site of transition metals and uncoordinated site (US) for catalyzing OER. The red and grayish-white spheres represent the O and Ru atoms, respectively. (C) 2D volcano of OER activity against the adsorption of *OOH (ΔG_*OOH) and *OH (ΔG_*OH). The black line is the scaling relation between ΔG_*OOH and ΔG_*OH, namely, ΔG_*OOH = 0.71ΔG_*OH + 2.89, R² = 0.58. The yellow, blue, and black fonts label the doped A, B, and C sites, respectively. A-TM, B-TM, C-TM, and A(TM) represent the Ru atoms adjacent to TM atoms in A, B, and C sites and the TM atom in the doped A site, respectively. The red font highlights the catalyst with the highest OER activity. (D) Bader charge of Ru sites adjacent to doped C sites. (E) DEMS signals of ³²O₂ (¹⁶O + ¹⁶O) and³⁴O₂ (¹⁶O + ¹⁸O) collected during the CV cycle. (F) Ratio of ³⁴O₂:³²O₂, which remained unchanged with the increase in potential, suggesting that ³⁴O₂ was derived from background ¹⁸O rather than from the lattice. (G) Free energy profile of OER on C-RuO₂(110) and C/A-RuO₂(110) with and without Mo adatoms. The inset is a comparison of activity (namely, overpotential) in C-RuO₂(110) and C/A-RuO₂(110).